Field of the Invention
The present invention relates to a process for recovering output of a gas sensor, and in particular to a process for recovering the output by heating a sensor element mainly composed of a solid electrolyte.
Description of the Background Art
Gas sensors that sense a predetermined gas component of a measurement gas, such as an exhaust gas, to determine its concentration come in various types such as semiconductor gas sensor, catalytic combustion gas sensor, oxygen-concentration difference sensing gas sensor, limiting current gas sensor, and mixed-potential gas sensor. Among these gas sensors, sensor elements mainly made of ceramic that is a solid electrolyte such as zirconia and including an electrode mainly made of a noble metal are widely known.
It is also known, in the gas sensors including the sensor elements mainly made of ceramic such as zirconia, that a gas component or a poisoning substance in a measurement gas is adhered to the surface of the electrode and therefore the output value varies, during the long time use. When the output varies, a recovering process is executed to the sensor elements to achieve the original (initial) output value or to obtain an output value as close to the original output value as possible. Examples of the recovering process include an electrical process (see, for example, Japanese Unexamined Patent Application Publication No. H6-265522 and Japanese Patent No. 3855979) and a heating process (see, for example, Japanese Unexamined Patent Application Publication No. H11-326266).
The electrical process is a method for recovering output by alternately applying positive and negative potentials between electrodes that are paired through a solid electrolyte, so as to refine the electrodes or to desorb an adsorbed substance.
Meanwhile, the heating process is a method for recovering output with exposure of an adsorbed substance or a poisoning substance to a high temperature to desorb or burn (oxidize) the substance.
The recovering of output through the heating process generally requires a long processing time. Once executing the heating process to a sensor element, the gas sensor can neither be measured for concentration nor function until the temperature of the sensor element is recovered to the normal operating temperature. Thus, the processing time for the heating process is preferred to be as short as possible.
However, the heating time disclosed in Japanese Unexamined Patent Application Publication No. H11-326266 is as long as approximately 10 minutes. Although Japanese Unexamined Patent Application Publication No. H11-326266 describes heating a sensing element up to a preset temperature, it fails to disclose or suggest shortening the heating time by optimizing a temperature profile of the heating.
Furthermore, when a sensing electrode of a sensor element in a hydrocarbon gas sensor contains a larger amount of Au (gold) whose melting point is lower, exposure of the sensor element to a high temperature environment for a long time through the heating process may alter the properties of the sensing electrode and influence output of the gas sensor. Also in this respect, the heating time is preferably as short as possible when the sensor element of the hydrocarbon gas sensor is to be recovered.